<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box{
            width: 100px;
            height: 100px;
            border: 2px solid red;
        }
        .item{
            width: 100px;
            height: 200px;
            background-color: skyblue;
            /* border-radius: 50px; */
            border-radius: 20%;
            /* 确定的是一个椭圆 
            水平半径=可视宽*20% 
                    = (width + 左右padding + 左右border) 

            水平半径=可视宽*20% 
                    = (width + 上下padding + 上下border) 
                    
                    
                    */
        }
    </style>
</head>
<body>
    <div class="box">
        <div class="item"></div>
    </div>
</body>
</html>